Rotary compressors are used in a variety of applications for compressing gases. As an example, with reference to FIG. 1, in turbochargers a rotating compressor wheel 11 within a compressor housing 13 sucks air through an intake port 15, compresses it in an impeller passage 17, and diffuses it into a volute 19. The compressed air is supplied to an intake manifold of an internal combustion engine. The operating range of a compressor extends from a surge condition, occurring at low airflow rates, to a choke condition experienced at high airflow rates. “Surging” occurs when a compressor operates at a relatively low flow rate with respect to the compressor pressure ratio, and the resulting flow of air throughout the compressor becomes unstable. “Choking” occurs when a compressor operates at a high flow rate that exceeds the mass flow rate available through the limited area of the intake end of the compressor wheel (known as the inducer) through which air arrives at the compressor wheel.
In order to improve the operating flow range, some compressors include one or more bypass ports 21 (such as in the form of an annular opening) on a compressor housing inner wall 23 (also referred to as a shroud) of the impeller passage 17 surrounding the compressor wheel 11. This “ported shroud” forms a shroud passageway 25 that extends between the bypass port(s) and a substantially annular opening 27 into the intake port 15 that feeds air in to the impeller passage. The ported shroud thus creates a second passageway connecting the intake port to the impeller passage, wherein this second passageway does not extend through the inducer.
The ported shroud typically improves the surge characteristics of a compressor by rerouting some air passing through the impeller passage back to the intake port during low-airflow operation, thereby extending the range over which the compressor can operate without experiencing a surge condition. The ported shroud may improve the choke characteristics of a compressor by providing an additional flow path into the impeller passage, without passing through the inducer, during high-airflow operation, thereby extending the range over which the compressor can operate without experiencing a choke condition.
While a ported shroud extends the operating range of a compressor, it also creates a systemic inefficiency. More particularly, the recirculated air that flows back to the intake port through the second passageway has been worked on by compressor wheel blades, and has been heated by the work done upon it. This heated recirculation flow increases the temperature of air entering the inducer, increasing the work needed from the turbine to compress the air, and thereby reducing the compressor efficiency.
Accordingly, there has existed a need for an apparatus and related methods to extend the flow range of a compressor without introducing significant inefficiencies from recirculated bypass air. Moreover, it is preferable that such apparatus are cost and weight efficient. Preferred embodiments of the present invention satisfy these and other needs, and provide further related advantages.